In a mobile communication system, establishing radio link connection is required between base stations and mobile terminals. A different plurality of frequencies included in a predetermined frequency bandwidth are employed as frequencies to be transmitted from a plurality of base stations. Each base station transmits an effective radio wave on a frequency selected from among these plurality of frequencies.
When the mobile terminal power is switched on, or when the mobile terminal moves outside a service area, in order to establish a radio link between the mobile terminal and a base station, the mobile terminal performs cell search for finding out where the effective radio wave is existent, namely, for searching a cell to which the radio link to be connected.
FIG. 1 shows a frame structure to be transmitted from the base station for performing this cell search. In FIG. 1, each radio frame length is 10 msec and has 15 slots (slot #0-#14). From each base station, one symbol of a common physical channel referred to as Synchronization Channel (SCH), which includes two kinds of synchronization channels (Primary SCH and Secondary SCH), having a length of 256 chips/symbol is transmitted in the top position of each slot at certain intervals.
It is known that the cell search in the W-CDMA system is performed according to the following three steps, using the above frame structure (Non-patent document, ‘W-CDMA Ido Tsushin Houshiki (Mobile Communication System)’ pp.35-37, supervised by Keiji Tachikawa, published by Maruzen Co., Ltd.).
First step=PSC: Detection of Primary Search Code
Second step=SSC: Detection of Secondary Search Code
Third step=SC: Detection of Scrambling Code
Namely, in the first step (=PSC), reception timing is detected using a system common code ACp of the Primary SCH. In the second step (=SSC), using 16 kinds of codes in the Secondary SCH, a scramble code group number (1-64) located in a certain position of each slot is identified, and the reception timing of the scrambling code is detected.
Next, in the third step (=SC), the scrambling code number is detected in Common Pilot Channel (CPICH) in which a known pilot pattern of 15 kbps is transmitted.
The above process is performed by switching frequencies, and a location registration is performed based on the decision of a cell from which the radio wave frequency producing the strongest intensity among the received radio waves is transmitted.
As inventions enabling the above band search processing, a variety of proposals have been disclosed up to the present. For example, one method is that selecting a bandwidth producing the maximum receiving power among the entire bandwidths, thereafter performing cell search by identifying the code, and deciding the bandwidth by the receiving signal code power (RSCP) (Japanese Patent Publication No. 2003-348648).
According to another invention, there has been disclosed a method of dividing a frequency bandwidth into a plurality of groups, selecting a group from the receiving power of a representative frequency, and deciding an area frequency by obtaining receiving power for each frequency (Japanese Patent Publication No. 2003-244083).